Aluminum and steel, i.e., "tin plated" DI cans are widely used in the manufacture of internally pressurized drink containers. The beverages contained by the DI cans include carbonated beverages, beer, and so forth.
The number of such cans produced each year is enormous and competition is intense. Generally, the cans are manufactured by a standard industrial process. In this process, prepared steel is either batch annealed or continuously annealed. The steel so used should have a particular hardness, defined by Rockwell T Hardness Standard HR30T (Hardness: 49-64), and a thickness of from 0.25-0.35 mm. The hardness standard is an industry-wide recognized one.
The steel sheet referred to here is tin plated, after which it is drawn and ironed. This material, now drawn and ironed, will be used to make the tin can. First the portion of the steel which will be the can edge is trimmed. Then, a flange is formed for seaming with an end of the can.
Generally, before flanging is carried out, the portion of the can that will be the can top is subjected to what is referred to as the "neck in" process. This results in shortening the diameter of the can top. The steps described herein require that the surface treated steel sheet to be used for DI cans possess excellent drawing formability, ironing workability, neck-in formability, flange formability and corrosion resistance. In addition, the process must be carried out in an economical fashion.
One of the approaches that have been taken to making the described process more economical is the manner of treating steel sheets to render them thin. It is necessary that the thinned sheets have high strength pressure resistance at the can bottom. Coupled with this is the need for good flange formability and drawability, as well as iron workability.
One approach to improving flange formability and making high strength material is shown in Japanese Tokukaishou (Laid-Open Patent Publication) No. 51-88415. This reference teaches improved flange formability (i.e., a reduction of crack occurrence ratio by several percent during flange formation), together with a steel sheet having cold rolled texture of more than 80%. This is accomplished by limiting the chemical composition of the steel. Specifically, the carbon quantity is kept to less than 0.02%, the sulphur quantity to less than 0.01%, and the Al/C ratio at more than 3.5.
The cracking referred to supra during flanging occurs because flanging requires widening the diameter of the can top. Also, the material at the end portion of the can shows poor ductility.
The flange crack occurrence ratio regarded by Tokakaishou 51-88415 as excellent, however, is not acceptable with the industry standard of about 10 part per million in batch or continuously annealed processes. Achieving a low flange crack occurrence ratio is one goal of the invention.